In ion mobility spectrometry devices, separation of gas-phase ions is accomplished by exploiting variations in ion drift velocities under an applied electric field arising from differences in ion mobility. One well-known type of ion mobility spectrometry device is the High Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS) cell, also known by the term Differential Ion Mobility Spectrometry (DMS) cell, which separates ions on the basis of a difference in the mobility of an ion at high field strength (commonly denoted as Kh) relative to the mobility of the ion at low field strength (commonly denoted as Kl). FAIMS cells may be used for a variety of purposes, including providing filtering of ions prior to entry into a mass analyzer.
Briefly described, a FAIMS cell or device comprises a pair of cylindrical electrodes separated by an annular space that defines an ion separation region through which ions are passed. During transit, ions in the separation gap experience an alternating electric field created using a bi-sinusoidal radio frequency (RF) waveform. The bi-sinusoidal waveform is a composite of a sine waveform and its scaled phase shifted harmonic. Ions oscillating between the high and low period of the bi-sinusoidal waveform can exert different electrical mobility. For instance, the electrical mobility of some ions can be higher during the high period of the waveform compared to the low period of the waveform. Ions of such mobility behavior in FAIMS are classed as A type ions and owing to their positive displacement over a single RF cycle, an A type ion can deviate away from the inner RF applied electrode and annihilate on the outer electrode over multiple RF cycles of the waveform. However, a DC offset (compensation voltage, or CV) can be applied to the electrode (inner bias) to correct for the trajectory of an A type ion such that the ion can be transmitted through the FAIMS device. Alternatively, a time-varying set of ions can be transmitted by sweeping the CV and generating a FAIMS spectrum ion current versus CV.